SkyFlux
Northern lights in Seattle
Seattle lies at geomagnetic latitude 53.0°. The aurora's oval has to reach that far south before anything appears above the northern horizon here — and that takes a Kp of about 6.5.
All three are ceilings, not promises. The night count says the Kp was reached at some point in those 24 hours — the peak still has to fall in your dark hours, and the sky still has to be clear. Halve it for cloud and you are nearer the truth.
Why 53.0° and not 47.61°
The aurora answers to the Earth's magnetic field, and the magnetic pole sits in northern Canada rather than at the geographic one. That tilt is worth several degrees: Seattle is at 47.61°N geographically and 53.0° magnetically, and it is the second number the oval reaches down to. The figure here is the standard dipole approximation. Measured against stations whose corrected geomagnetic latitude is published, it reads about 0.6° too far poleward — roughly a third of a Kp step, and it errs on the optimistic side. We would rather write that down than round it away.
How often that actually happens
Over the last 30 years — solar cycles 23, 24 and 25 so far — the planetary Kp index reached 6.7 on about 6 days a year, which is 2% of all days. At this latitude 6 of them fall in a part of the year when the sky can go astronomically dark at all.
A Kp of 6.7 means a geomagnetic storm, not ordinary auroral activity: Kp is reported in thirds, and a reading of 6.7 counts as level G3 on NOAA's five-level storm scale. Aurora in Seattle is something to watch for when a storm is forecast, not something an ordinary clear night brings.
That figure is measured rather than quoted. NOAA's published storm scale puts a Kp 5 storm at 900 days per solar cycle, about 82 days a year, and that is right for the cycles it was drawn from. It is not right for ours: the record gives 48 days a year for cycle 23, 25 for cycle 24 — the weakest on record — and 34 for cycle 25 so far. Quoting the table would have inflated every number on this page two to three times over.
The other half of the answer is darkness
A storm you cannot see is not an event, but here the time of year hides none of them. Seattle is far enough south that the sun gets 18° below the horizon every night of the year, so there is no summer stretch when twilight keeps the sky too bright: every day the Kp reaches its level falls on a night that can go astronomically dark, and nothing is taken off the Kp count for it. Of the two measurements on this page, only the Kp limits aurora in Seattle; cloud and the hour of the peak still apply, as everywhere.
What the app does with this
SkyFlux runs the same calculation for wherever you are actually standing, not for the nearest city on a list, and puts the live Kp forecast against it. It also tells you when the sky goes dark tonight, what the cloud is doing hour by hour, and where the nearest genuinely dark ground is. No account, no ads, and the arithmetic runs on the phone.
Other places
Kp index: GFZ Helmholtz Centre for Geosciences, Geomagnetic Observatory Niemegk, CC BY 4.0 (Matzka et al. 2021). Storm scale and the 66°/2° rule: NOAA Space Weather Prediction Center. Geomagnetic latitude: dipole approximation. Darkness: solar altitude at lower culmination, −18°.